Receptor tyrosine kinases (RTKs) have been implicated in cellular signaling pathways that control various cellular functions, including cell division, growth, metabolism, differentiation, and survival, through reversible phosphorylation of the hydroxyl groups of tyrosine residues in proteins. Extracellular signals are transduced via activation of the cell surface receptors, with amplification and propagation using a complex choreography of cascades of protein phosphorylation, and protein dephosphorylation events to avoid uncontrolled signaling. These signaling pathways are highly regulated, often by complex and intermeshed kinase pathways where each kinase may itself be regulated by one or more other kinases and protein phosphatases. The biological importance of these finely tuned systems is such that a variety of cell proliferative disorders have been linked to defects in one or more of the various cell signaling pathways mediated by tyrosine or serine/threonine kinases.
Receptor tyrosine kinases (RTKs) catalyse phosphorylation of certain tyrosyl amino acid residues in various proteins, including themselves, which govern cell growth, proliferation and differentiation. Insulin-like growth factor-1 receptor (IGF-1R) is a transmembrane tyrosine kinase ubiquitous among fetal and post-natal cell types.
The IGF signaling axis is made up of multiple ligands (IGF-1, IGF-2 and Insulin), at least six high affinity ligand binding proteins and proteases, multiple receptors (IGF-1R & IGF-2R, IR and IRR), and many other down stream signaling proteins (Pollak, M N et al., Nature Reviews Cancer (2004) 4(7):505-518). The structure and function of the IGF-1R has been reviewed by Adams et al., Cell. Mol. Life Sci. (2000) 57:1050-1093 and Benito, M et al., Int J Biochem Cell Biol (1996) 28(5):499-510. The receptor is activated by the ligands IGF-1 and IGF-2, which are mitogenic proteins that signal through the IGF-1R and IR in an endocrine, paracrine or autocrine manner. Activation of the IGF-1 receptor elicits cellular responses which include cellular proliferation and protection of cells from apoptosis. (Id.) Overexpression of IGF-1R leads to malignant transformation of cultured cells, while downregulation can reverse the transformed phenotype of tumor cells and potentially render them susceptible to apoptosis. (Id.)
There are two splice variants of the IR gene, the IR-B isoform which regulates glucose uptake and is expressed in liver, muscle and adipose tissue, and the exon 11 variant human insulin receptor isoform A (IR-A) binds IGF-2 with high affinity and promotes proliferation and protection from apoptosis (Sciacca L. Oncogene (2002) 21(54):8240-8250). IR-A is predominantly expressed in fetal tissue and malignancies and at this receptor, IGF-2 is more potent than insulin in stimulating cancer cell migration. (Sciacca, Oncogene (2002) supra). Insulin receptor-related receptor (IRR) has 79% homology with the kinase domain of IR and is expressed only in a few limited cell types (Dandekar, A A et al., Endocrinology (1998) 139(8):3578-3584).
IGF-1R is a hetero-tetrameric, transmembrane, cell surface receptor. (Benito, Int J Biochem Cell Biol (1996)) An IGF-1 binding domain is part of the extracellular alpha-chain of IGF-1R, whereas the intracellular beta-chain contains the tyrosine kinase domain. Three tyrosine residues represent autophosphorylation sites, specifically Tyr1131, Tyr1135, and Tyr1136, within the activation loop of the IGF-1R beta catalytic domain (Li, W et al., J. Biol. Chem. (2006) 281(33):23785-23791). Phosphorylation of all three is required for full receptor activation, and preceeds phosphorylation of juxtamembrane tyrosines and carboxy terminus serines. The insulin receptor has three similar autophosphorylation sites on the activation loop and juxtamembrane region. Activation and autophoshorylation results in the recruitment of multiple docking proteins and the generation of intracellular signaling (Benito, Int J Biochem Cell Biol (1996)). Once activated, IGF-1R and IR can phosphorylate or interact directly with a number of intracellular protein substrates, including IRS-1, IRS-2, grb2, grb10, grb14, Shc, SOC, 14.3.3, FAK, or indirectly with other proteins like PI3K and MAPK (Benito, M et al. Int J Biochem Cell Biol (1996) 28(5):499-510) (Brown, G C et al., Biochem. J (1992) 284:1-13; Bruning, J C et al., Mol. Cell (1998) 2(5):559-569). Focal adhesion kinase (FAK) is of particular interest because of its role as a regulator of cell survival, proliferation, migration and invasion. FAK is activated by growth factor receptors such as IGF-1R, by binding through its N-terminal domain and autophosphorylation at TyR397. Activated or over expressed FAK is common in a wide variety of cancers, and may play a role in human carcinogenesis (van Nimwegen, M J et al., Biochem. Pharmacol. (2007) 73(5):597-609).
In addition to its role in cancers, the insulin-like growth factor receptor plays important and diverse roles in growth and development (Benito, M et al. Int J Biochem Cell Biol (1996) 28(5):499-510). It IGF-1R has been implicated in several metabolic, and immunological diseases (Walenkamp, M J et al., Horm. Res. (2006) 66(5):221-230; Kurmasheva, R. T et al., Biochim. Biophys. Acta—Rev on Cancer (2006) 1766(1):1-22; Bateman, J M et al., Cell. Mol. Life Sci. (2006) 63(15):1701-1705, LeRoith, D, et al., Can. Lett. (2003) 195:127-137 and Samani A, et al., Endocrine Reviews 28(1):20-47.)
The role of the IGF/IGF-1R signaling system in cancer has been thoroughly examined over the last 15 years. In particular, the implication of IGF-1R in human cancer stems from its roles in stimulating mitogenesis, mobility and metastasis and in protecting against apoptosis. (Kurmasheva, Biochim. Biophys. Acta (2006).) Interest has grown with the understanding that in addition to its antiapoptotic and mitogenic roles, IGF/IGF-1R signaling seems to be necessary for the establishment and continuation of a transformed phenotype. It has been well established that constitutive activation or over expression, often results in non-adherent cell growth, even under serum depleted conditions in vitro, and is associated with the formation of tumors in nude mice. (Kaleko M et al, Mol Cell Biol. (1990) 10(2): 464-473). Perhaps even more importantly, it has been firmly established that cells, in which the gene encoding for IGF-1R has been deactivated, are totally resistant to transformation by agents which are normally capable of immortalizing normal cells, such as over expression of PDGFR or EGFR, the T antigen of the SV40 virus, the E5 protein of bovine papilloma virus, and activated ras. (DeAngelis T et al., Cell. Physiol. (1995) 164( ):214-221; Coppola D et al., Mol. Cell. Biol. (1994) 14(7):4588-4595; Morrione A J, Virol. 1995 695300-5303; Sell C et al., Mol. Cell. Biol. (1994) 14(6):3604-3612; Sell C et al., Proc. Natl. Acad. Sci. USA (1993) 90(23):11217-11221). Thus, IGF-1R has been identified as the major survival factor that protects from oncogene induced cell death (Harrington et al., EMBO J. (1994) 13( ):3286-3295). IGF-1R is expressed in a large number and variety of tumors and the IGFs amplify the tumor growth through their interaction with the receptor. Evidence supporting the role of IGF-1R in carcinogenesis can be found in studies using monoclonal antibodies directed towards the receptor which inhibit the proliferation of numerous cell lines in culture and in vivo (Arteaga C et al., Cancer Res. (1989) 49(22):6237-6241; Li et al., Biochem. Biophys. Res. Com. (1993) 196(1):92-98; Scotlandi K et al., Cancer Res. (1998) 58(18):4127-4131). Dominant negative IGF-1R is capable of inhibiting tumor proliferation (Jiang et al., Oncogene (1999) 18(44):6071-6077).
The IGF signaling axis is implicated in various tumor types including: breast cancer (Surmacz, J. Mammary Gland Bio. Neoplasia (2000) 5(1):95-105, LeRoith, Can. Lett. (2003) and Artega, Cancer Res. (1989)), bone and bone marrow cancers including Ewing's sarcoma, osteosarcoma, giant cell tumor of bone (Scotlandi, Cancer Res. (1998) lung cancer, including non-small cell and small cell lung carcinomas and mesotheliomas (Jiang, Y et al., Oncogene (1999) 18:6071-6077 and LeRoith, Can. Lett. (2003), prostate cancer (Djavan et al., World J Urol. (2001) 19(4):225-233; O'Brien et al., Urology (2001) 58(1):1-7 and LeRoith, Can. Lett. (2003)), colorectal cancer (Guo et al., Gastroenterology, 1992, 102, 1101-1108; Durai, R et al., Int. J Colorectal Dis. (2005) 20(3):203-220 and LeRoith, Can. Lett. (2003)), renal cancer (Kellerer M. et al., Int. J. Cancer (1995) 62(5):501-507), pancreatic cancer (Bergmann, U et al., Cancer Res. (1995) 55(10):2007-2011), hematopoietic cancers, including lymphoblastic T cell leukemia, chronic myelogenous leukemia, chronic lymphocytic leukemia, hairy-cell leukemia, acute lymphoblastic leukemia, acute myelogenous leukemia, chronic neutrophilic leukemia, acute lymphoblastic T cell leukemia, plasmacytoma, immunoblastic large cell leukemia, mantle cell leukemia, multiple myeloma, megakaryoblastic leukemia, acute megakaryocytic leukemia, promyelocytic leukemia, erythroleukemia, malignant lymphoma, Hodgkins lymphoma, non-Hodgkins lymphoma, lymphoblastic T cell lymphoma, Burkitt's lymphoma, follicular lymphoma, myelodysplastic syndromes, (Zumkeller W et al., Leuk. Lymph (2002) 43(3):487-491; and Qi, Ann Hematol. (2006) 85:95-101.), neuroblastomas (Zumkeller, W et al., Horm. Metab. Res. 1999, 31, 138-141), gliomas, meningiomas, medulloblastomas, astrocytomas, and glioblastoma (Zumkeller, Wet al., Mol. Pathol. (2001) 54(4):227-229, Del Valle L, et al., Clin. Cancer Res. (2002) 8:1822-1830 and Trojan et al., Proc. Natl. Acad. Sci. U.S.A. (1992) 89:4874-4878.), thyroid cancer (Vella V et al., J. Clin. Endocrinol. Metab. (2002) 87:245-254; Vella V et al., Mol. Pathol. (2001) 54(3):121-124), and hepatocarcinoma (Alexia, C et al., Biochem. Pharmacol. (2004) 68(1):1003-1015). ovarian cancer, testicular cancer, vulval cancer, cervical cancer, endometrial cancer, bladder cancer, esophageal cancer, gastric cancer, nasopharangeal cancer, buccal cancer, cancer of the mouth, GIST (gastrointestinal stromal tumor), skin cancer including melanoma, and soft-tissue sarcoma.
Thus, in virtually all types of human cancers there is a strong association between dysregulation of IGF signaling and carcinogenesis (Bohula E A et al., Anticancer Drugs (2003) 14(9):669-682). Inhibition of IGF-1R expression or function has been shown to block tumor growth and metastasis and also enhance sensitivity to other anti-neoplastic therapies, including cytotoxic drugs and radiation. (Bohula, Anticancer Drugs (2003).